An observation based on what and/or who? The counter-examples in this case are numerous:
Tesla http://en.wikipedia.org/wiki/Nikola_Tesla Feynman http://en.wikipedia.org/wiki/Richard_Feynman
Tesla http://en.wikipedia.org/wiki/Nikola_Tesla Feynman http://en.wikipedia.org/wiki/Richard_Feynman
"Age is another factor which the physicists particularly worry about. They always are saying that you have got to do it when you are young or you will never do it. Einstein did things very early, and all the quantum mechanic fellows were disgustingly young when they did their best work. Most mathematicians, theoretical physicists, and astrophysicists do what we consider their best work when they are young. It is not that they don't do good work in their old age but what we value most is often what they did early. On the other hand, in music, politics and literature, often what we consider their best work was done late. I don't know how whatever field you are in fits this scale, but age has some effect."
I'm sure there are plenty of counter-examples, yes.